1. Field of the Invention
The present disclosure relates to a color image forming apparatus that can perform color registration adjustment.
2. Description of the Related Art
There is a tandem image forming apparatus that forms a color image by superimposing toner images formed by image forming units provided for respective colors. In such a tandem image forming apparatus, when the toner images of the respective colors are subjected to multilayer transfer, the image forming positions of the image forming units, including photosensitive members and exposure devices (laser scanners), may shift due to an initial installation state, a change over time, or a temperature change. The shifts in the image forming positions of the image forming units cause the misregistration of the images of the respective colors.
To prevent the formation of an image having the misregistration of the images of the respective colors, various color registration adjustment methods are proposed. A color registration pattern is formed using respective image forming units and read by a sensor, thereby detecting the amount of color misregistration. Then, the timing of forming the image of each color is adjusted based on the amount of color misregistration.
Further, there is proposed a technique of predicting the amount of color misregistration from the amount of temperature change without forming a color registration pattern.
Japanese Patent Application Laid-Open No. 2006-11289 discusses a technique of predicting the amount of color misregistration in a sub-scanning direction using a temperature sensor in the housing of an exposure device, and adjusting the timing of scanning.
Further, Japanese Patent Application Laid-Open No. 2007-108283 and U.S. Pat. No. 8,270,857 discuss a technique of correcting the amount of color misregistration by referring to a prediction table based on a temperature change detected by a temperature sensor in an image forming apparatus. Further, if the absolute value of the amount of temperature change in the image forming apparatus is equal to or greater than a threshold, the image forming apparatus forms and measures a color registration pattern. Then, the amount of color misregistration is calculated based on the measurement result. Then, the prediction table is corrected based on the calculated amount of color misregistration and the amount of temperature change at that time.
Recently, however, an image quality required by the market is increasingly heightened. The methods of predicting the amount of color misregistration based only on the amount of change in temperature of an exposure device (laser scanner) or the amount of change in temperature inside an image forming apparatus as in the conventional arts are not sufficient. It is very difficult to predict the amount of color misregistration with high accuracy using only one temperature sensor. It is not possible to sufficiently deal with the influence of a hysteresis due to a rise or fall in temperature inside the image forming apparatus.